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docs:guide-user:base-system:dhcp

DNS and DHCP configuration /etc/config/dhcp

See also: DNS and DHCP configuration examples

OpenWrt uses dnsmasq and odhcpd to serve DHCP and DNS by default.

Software Roles Ports Configuration
dnsmasq DNS 53/TCP, 53/UDP /etc/config/dhcp
DHCP 67/UDP
odhcpd DHCPv6 547/UDP

In the default configuration this file contains one common section to specify DNS and daemon related options and one or more DHCP pools to define DHCP serving on network interfaces.

Sections

Possible section types of the dhcp configuration file are defined below. Not all types may appear in the file and most of them are only needed for special configurations. The common ones are the Common Options, the DHCP Pools and Static Leases.

Common options

Sections of the type dnsmasq specify per dnsmasq instance the values and options relevant to the overall operation of the dnsmasq instance and the DHCP options on all interfaces served. The following table lists all available options, their default value, as well as the corresponding dnsmasq command line option. See the dnsmasq man page for further details.

These are the default settings for the common options:

# uci -N show dhcp.@dnsmasq[0]
dhcp.@dnsmasq[0]=dnsmasq
dhcp.@dnsmasq[0].domainneeded='1'
dhcp.@dnsmasq[0].boguspriv='1'
dhcp.@dnsmasq[0].filterwin2k='0'
dhcp.@dnsmasq[0].localise_queries='1'
dhcp.@dnsmasq[0].rebind_protection='1'
dhcp.@dnsmasq[0].rebind_localhost='1'
dhcp.@dnsmasq[0].local='/lan/'
dhcp.@dnsmasq[0].domain='lan'
dhcp.@dnsmasq[0].expandhosts='1'
dhcp.@dnsmasq[0].nonegcache='0'
dhcp.@dnsmasq[0].authoritative='1'
dhcp.@dnsmasq[0].readethers='1'
dhcp.@dnsmasq[0].leasefile='/tmp/dhcp.leases'
dhcp.@dnsmasq[0].resolvfile='/tmp/resolv.conf.auto'
dhcp.@dnsmasq[0].nonwildcard='1'
dhcp.@dnsmasq[0].localservice='1'

Options:

  • local and domain enable dnsmasq to serve entries in /etc/hosts, as well as DHCP client's names if configured under lan domain.
  • domainneeded, boguspriv, localise_queries, and expandhosts ensure requests for local host names are not forwarded to upstream DNS servers.
  • authoritative makes router the only DHCP server on this network; clients get their IP lease a lot faster this way.
  • leasefile stores leases in a file so they can be picked up again if dnsmasq is restarted.
  • resolvfile tells dnsmasq to use this file to find upstream name servers; it gets created by the WAN DHCP or PPP client.
  • enable_tftp and tftp_root turn on the TFTP server and serve files from tftp_root.
    • May need to set server's IP on client, changing it by setting serverip (e.g. setenv serverip 192.168.1.10).

All options

Name Type Default Option Description
add_local_domain boolean 1 Add the local domain as search directive in resolv.conf.
add_local_hostname boolean 1 Add A, AAAA, and PTR records for this router only on DHCP served LAN.
:!: enhanced function available on Trunk with option add_local_fqdn
add_local_fqdn integer 1 Add A, AAAA, and PTR records for this router only on DHCP served LAN.
:!: add_local_fqdn on Trunk but not 17.01.0
0: Disable.
1: Hostname on Primary Address.
2: Hostname on All Addresses.
3: FDQN on All Addresses.
4: iface.host.domain on All Addresses.
:!: add_local_fqdn on Trunk but not 17.01.0
add_wan_fqdn integer 0 Labels WAN interfaces like add_local_fqdn instead of your ISP assigned default which may be obscure. WAN is inferred from config dhcp sections with option ignore 1 set, so they do not need to be named WAN
:!: add_wan_fqdn on Trunk but not 17.01.0
addnhosts list of file paths (none) -H Additional host files to read for serving DNS responses. Syntax in each file is the same as /etc/hosts
authoritative boolean 1 -K Force dnsmasq into authoritative mode. This speeds up DHCP leasing. Used if this is the only server on the network
bogusnxdomain list of IP addresses (none) -B IP addresses to convert into NXDOMAIN responses (to counteract “helpful” upstream DNS servers that never return NXDOMAIN).
boguspriv boolean 0 -b Reject reverse lookups to private IP ranges where no corresponding entry exists in /etc/hosts
cachelocal boolean 1 When set to 0, use each network interface's dns address in the local /etc/resolv.conf. Normally, only the loopback address is used, and all queries go through dnsmasq.
cachesize integer 150 -c Size of dnsmasq query cache.
dbus boolean 0 -1 Enable DBus messaging for dnsmasq.
:!: Standard builds of dnsmasq on OpenWrt do not include DBus support.
dhcp_boot string (none) --dhcp-boot Specifies BOOTP options, in most cases just the file name. You can also use: “file name, tftp server name, tftp ip address
dhcphostsfile file path (none) --dhcp-hostsfile Specify an external file with per host DHCP options
dhcpleasemax integer 150 -X Maximum number of DHCP leases
dnsforwardmax integer 150 -0 (zero) Maximum number of concurrent connections
domain domain name (none) -s DNS domain handed out to DHCP clients
domainneeded boolean 1 -D Tells dnsmasq never to forward queries for plain names, without dots or domain parts, to upstream nameservers. If the name is not known from /etc/hosts or DHCP then a “not found” answer is returned
dnssec boolean 0 --dnssec Validate DNS replies and cache DNSSEC data.
:!: Requires the dnsmasq-full package.
dnsseccheckunsigned boolean 0 --dnssec-check-unsigned Check the zones of unsigned replies to ensure that unsigned replies are allowed in those zones. This protects against an attacker forging unsigned replies for signed DNS zones, but is slower and requires that the nameservers upstream of dnsmasq are DNSSEC-capable.
:!: Requires the dnsmasq-full package.
:!: Caution: If you use this option on a device that doesn't have a hardware clock, dns resolution may break after a reboot of the device due to an incorrect system time.
ednspacket_max integer 1280 -P Specify the largest EDNS.0 UDP packet which is supported by the DNS forwarder
enable_tftp boolean 0 --enable-tftp Enable the builtin TFTP server
expandhosts boolean 1 -E Add the local domain part to names found in /etc/hosts
filterwin2k boolean 0 -f Do not forward requests that cannot be answered by public name servers.
Make sure it is disabled if you need to resolve SRV records or use SIP phones.
fqdn boolean 0 --dhcp-fqdn Do not resolve unqualifed local hostnames. Needs domain to be set.
interface list of interface names (all interfaces) -i List of interfaces to listen on. If unspecified, dnsmasq will listen to all interfaces except those listed in notinterface. Note that dnsmasq listens on loopback by default.
ipset list of strings (none) --ipset The syntax is: list 'ipset' '/example.com/example.org/example_ipv4,example_ipv6'
:!: Requires the dnsmasq-full package.
leasefile file path (none) -l (lowercase “L”) Store DHCP leases in this file
local string (none) -S Look up DNS entries for this domain from /etc/hosts. This follows the same syntax as server entries, see the man page.
localise_queries boolean 0 -y Choose IP address to match the incoming interface if multiple addresses are assigned to a host name in /etc/hosts. :!: Note well the spelling of this option.
localservice boolean 1 --local-service Accept DNS queries only from hosts whose address is on a local subnet, ie a subnet for which an interface exists on the server.
localuse boolean 1 Use dnsmasq as a local system resolver. Although localuse is initially disabled, it is still enabled via the default noresolv and resolvfile options.
logqueries boolean 0 -q Log the results of DNS queries, dump cache on SIGUSR1
nodaemon boolean 0 -d Don't daemonize the dnsmasq process
nohosts boolean 0 -h Don't read DNS names from /etc/hosts
nonegcache boolean 0 -N Disable caching of negative “no such domain” responses
noresolv boolean 0 -R Don't read upstream servers from /etc/resolv.conf which is linked to resolvfile by default
notinterface list of interface names (none) -I (uppercase “i”) Interfaces dnsmasq should not listen on.
nonwildcard boolean 1 --bind-dynamic Bind only configured interface addresses, instead of the wildcard address.
port port number 53 -p Listening port for DNS queries, disables DNS server functionality if set to 0
queryport integer (none) -Q Use a fixed port for outbound DNS queries
readethers boolean 0 -Z Read static lease entries from /etc/ethers, re-read on SIGHUP
rebind_protection boolean 1 --stop-dns-rebind Enables DNS rebind attack protection by discarding upstream RFC1918 responses
rebind_localhost boolean 0 --rebind-localhost-ok Allows upstream 127.0.0.0/8 responses, required for DNS based blacklist services, only takes effect if rebind protection is enabled
rebind_domain list of domain names (none) --rebind-domain-ok List of domains to allow RFC1918 responses for, only takes effect if rebind protection is enabled. The correct syntax is: list rebind_domain '/example/'
resolvfile file path /tmp/resolv.conf.auto -r Specifies an alternative resolv file
server list of strings (none) -S List of DNS servers to forward requests to. See the dnsmasq man page for syntax details.
rev_server list of strings (none) --rev-server List of network range with a DNS server to forward reverse DNS requests to. See the dnsmasq man page for syntax details.
strictorder boolean 0 -o Obey order of DNS servers in /etc/resolv.conf
tftp_root directory path (none) --tftp-root Specifies the TFTP root directory
minport integer 0 --min-port Dnsmasq picks random ports as source for outbound queries. When this option is given, the ports used will always be larger than or equal to the specified minport value (min valid value 1024). Useful for systems behind firewalls.
maxport integer 0 --max-port Dnsmasq picks random ports as source for outbound queries. When this option is given, the ports used will always be smaller than or equal to the specified maxport value (max valid value 65535). Useful for systems behind firewalls.
noping boolean 0 --no-ping By default dnsmasq checks if an IPv4 address is in use before allocating it to a host by sending ICMP echo request (aka ping) to the address in question. This parameter allows to disable this check.
allservers boolean 0 --all-servers By default, when dnsmasq has more than one upstream server available, it will send queries to just one server. Setting this parameter forces dnsmasq to send all queries to all available servers. The reply from the server which answers first will be returned to the original requeser.
quietdhcp boolean 0 --quiet-dhcp Suppress logging of the routine operation of DHCP. Errors and problems will still be logged
sequential_ip boolean 0 --dhcp-sequential-ip Dnsmasq is designed to choose IP addresses for DHCP clients using a hash of the client's MAC address. This normally allows a client's address to remain stable long-term, even if the client sometimes allows its DHCP lease to expire. In this default mode IP addresses are distributed pseudo-randomly over the entire available address range. There are sometimes circumstances (typically server deployment) where it is more convenient to have IP addresses allocated sequentially, starting from the lowest available address, and setting this parameter enables this mode. Note that in the sequential mode, clients which allow a lease to expire are much more likely to move IP address; for this reason it should not be generally used.
addmac [0,1,base64,text] 0 --add-mac Add the MAC address of the requester to DNS queries which are forwarded upstream; this may be used to do DNS filtering by the upstream server.
The MAC address can only be added if the requester is on the same subnet as the dnsmasq server. Note that the mechanism used to achieve this (an EDNS0 option) is not yet standardised, so this should be considered experimental. Also note that exposing MAC addresses in this way may have security and privacy implications.
logdhcp boolean 0 --log-dhcp Enables extra DHCP logging; logs all the options sent to the DHCP clients and the tags used to determine them

DHCP pools

Sections of the type dhcp specify per interface lease pools and settings for serving DHCP requests. Typically there is at least one section of this type present in the /etc/config/dhcp file to cover the lan interface.

You can disable a lease pool for a specific interface by specifying the ignore option in the corresponding section.

A minimal example of a dhcp section is listed below:

# uci -N show dhcp.@dhcp[0]
dhcp.@dhcp[0]=dhcp
dhcp.@dhcp[0].interface='lan'
dhcp.@dhcp[0].start='100'
dhcp.@dhcp[0].limit='150'
dhcp.@dhcp[0].leasetime='12h'
  • lan specifies the OpenWrt interface that is served by this DHCP pool
  • 100 is the offset from the network address, in the default configuration this would mean start leasing addresses from 192.168.1.100
  • 150 is the maximum number of addresses that may be leased, in the default configuration this would mean leasing addresses up to 192.168.1.250
  • 12h specifies the time to live for handed out leases, twelve hours in this example
  • server defines the mode for IPv6 configuration (RA & DHCPv6)

Below is a listing of legal options for dhcp sections.

Name Type Required Default Description
dhcp_option list of strings no (none) The ID dhcp_option here must be with written with an underscore. OpenWrt will translate this to --dhcp-option, with a hyphen, as ultimately used by dnsmasq. Multiple option values can be given for this network-id, with a a space between them and the total string between ””. E.g. '26,1470' or 'option:mtu, 1470' that can assign an MTU per DHCP. Your client must accept MTU by DHCP for this to work. Or “3,192.168.1.1 6,192.168.1.1” to give out gateway and dns server addresses. A list of options can be found here here.
dhcp_option_force list of strings no (none) Exactly the same as dhcp_option (note the underscores), but it will be translated to --dhcp-option-force, meaning that the DHCP option will be sent regardless on whether the client requested it.
:!: dhcp_option_force on Trunk but not 17.01.0
dynamicdhcp boolean no 1 Dynamically allocate client addresses, if set to 0 only clients present in the ethers files are served
force boolean no 0 Forces DHCP serving on the specified interface even if another DHCP server is detected on the same network segment
ignore boolean no 0 Specifies whether dnsmasq should ignore this pool if set to 1
dhcpv4 string no (none) Specifies whether DHCPv4 server should be enabled (server) or disabled (disabled)
dhcpv6 string no (none) Specifies whether DHCPv6 server should be enabled (server), relayed (relay) or disabled (disabled)
ra string no (none) Specifies whether Router Advertisements should be enabled (server), relayed (relay) or disabled (disabled)
ra_default integer no 0 Default router lifetime in the RA message will be set if default route is present and a global IPv6 address (0) or if default route is present but no global IPv6 address (1) or neither of both conditions (2)
ra_management integer no 1 RA management mode : no M-Flag but A-Flag (0), both M and A flags (1), M flag but not A flag (2)
ra_offlink bool no 0 Announce prefixes as offlink (1) in RAs
ra_preference string no medium Announce routes with either high (high), medium (medium) or low (low) priority in RAs
ra_mininterval integer no 200 Minimum time interval between RAs (in seconds)
ra_maxinterval integer no 600 Maximum time interval between RAs (in seconds)
ra_lifetime integer no 1800 Advertised router lifetime (in seconds)
ra_useleasetime bool no 0 Limit the preferred and valid lifetimes of the prefixes in the RA messages to the configured DHCP leasetime
ra_hoplimit integer no 0 Advertised current hop limit (0-255)
ra_reachabletime integer no 0 Advertised reachable time (in milliseconds) (0-3600000)
ra_retranstime integer no 0 Advertised NS retransmission time (in milliseconds) (0-60000)
ra_mtu integer no (none) Maximum advertised MTU
ndp string no (none) Specifies whether NDP should be relayed (relay) or disabled (disabled)
ndproxy_routing bool no 1 Learn routes from NDP
ndproxy_slave bool no 0 Ignore neighbor messages on slave enabled (1) interfaces
master boolean no 0 Specifies whether DHCPv6, RA and NDP in relay mode is a master interface or not.
interface logical interface name yes (none) Specifies the interface associated with this DHCP address pool; must be one of the interfaces defined in /etc/config/network.
leasetime string yes 12h Specifies the lease time of addresses handed out to clients, for example 12h or 30m
limit integer yes 150 Specifies the size of the address pool (e.g. with start=100, limit=150, maximum address will be .249)
networkid string no (value of interface) The dhcp functionality defined in the dhcp section is limited to the interface indicated here through its network-id. In case omitted the system tries to know the network-id via the interface setting in this dhcp section, through consultation of /etc/config/network. Some IDs get assigned dynamically, are not provided by network, but still can be set here.
start integer yes 100 Specifies the offset from the network address of the underlying interface to calculate the minimum address that may be leased to clients. It may be greater than 255 to span subnets.
instance dnsmasq instance no (none) Dnsmasq instance to which the dhcp section is bound; if not specified the section is valid for all dnsmasq instances.
tag list of tag names no (none) List of tags that dnsmasq needs to match to use with --dhcp-range.

Notes:

  • interface is a logical interface / network name, i.e. lan, wan, wifi etc. (section names in /etc/config/network), NOT a layer 3 device name like eth0, eth1, wlan0 etc. (the ifname IDs in /etc/config/network).
  • networkid is a layer 3 device name, i.e. eth0, eth1, wlan0 etc., not a network name (lan, wan, wifi etc.).

This departs from ifname and network as used in /etc/config/network and in /etc/config/wireless, so double check!

Static leases

You can assign fixed IP addresses to hosts on your network, based on their MAC (hardware) address. The configuration options in this section are used to construct a -G option for dnsmasq.

Name Type Required Default Description
ip string yes (none) 'ignore' or the IP address to be used for this host.
mac string no (none) The hardware address(es) of this host, separated by commas.
hostid string no (none) The IPv6 interface identifier (address suffix) as hexadecimal number (max. 8 chars)
duid string no (none) The DHCPv6-DUID of this host.
name string no (none) Optional hostname to assign.
tag string no (none) Set the given tag for matching hosts.
dns boolean no 0 Add static forward and reverse DNS entries for this host.
broadcast boolean no 0 Force broadcast DHCP response.
leasetime string no (none) Host-specific lease time, e.g. 2m, 3h, 5d. Note: introduced by r48801 in trunk
instance dnsmasq instance no (none) Dnsmasq instance to which the host section is bound; if not specified the section is valid for all dnsmasq instances.

As well as adding host sections, you can also enable the dnsmasq section option readethers, and add entries to the /etc/ethers file.

Booting options

Some hosts support booting over the network (PXE booting). DHCP/BOOTP is used to tell the host which file to boot and the server to load it from. Each client can only receive one set of filename and server address options. If different hosts should boot different files, or boot from different servers, you can use network-ids to map options to each client.

Usually, you need to set additional DHCP options (through dhcp_option) for further stages of the boot process. See the dnsmasq man page for details on the syntax of the O option.

The configuration options in this section are used to construct a -M option for dnsmasq.

*Note*: odhcp currently lacks support root-path specification. If you need this functionality, disable odhcpd and use dnsmasq instead.

Name Type Required Default Description
dhcp_option list of strings no (none) Additional options to be added for this network-id. :!: If you specify this, you also need to specify the network-id.
filename string yes (none) The filename the host should request from the boot server.
networkid string no (none) The network-id these boot options should apply to. Applies to all clients if left unspecified.
serveraddress string yes (none) The IP address of the boot server.
servername string yes (none) The hostname of the boot server.
force bool no (none) dhcp_option will always be sent, even if the client does not ask for it in the parameter request list. This is sometimes needed, for example when sending options to PXELinux.
instance dnsmasq instance no (none) Dnsmasq instance to which the boot section is bound. If not specified the section is valid for all dnsmasq instances.

Classifying clients and assigning individual options

DHCP can provide the client with numerous options, such as the domain name, NTP servers, network booting options, etc. While some settings are applicable to all hosts in a network segment, other are more specific and apply only to a group of hosts, or even only a single one. dnsmasq offers to group DHCP options and their values by a network-id, an alphanumeric identifier, and sending options only to hosts which have been tagged with that network-id.

In OpenWrt, you can tag hosts by the DHCP range they're in (section dhcp), or a number of options the client might send with their DHCP request. In each of these sections, you can use the dhcp_option list to add DHCP options to be sent to hosts with this network-id.

You can use the following classifying sections:

Name Description
mac Hardware address of the client.
tag An alphanumeric label which marks the network.
vendorclass String sent by the client representing the vendor of the client. dnsmasq performs a substring match on the vendor class string using this value.
userclass String sent by the client representing the user of the client. dnsmasq performs a substring match on the user class string using this value.
circuitid Matches the circuit ID as sent by the relay agent, as defined in RFC3046.
remoteid Matches the remote ID as sent by the relay agent, as defined in RFC3046.
subscrid Matches the subscriber ID as sent by the relay agent, as defined in RFC3993.

Each classifying section has two configuration options: the value of the DHCP option used to distinguish clients, and the network-id that these clients should be tagged with.

Name Type Required Default Description
classifier string yes (none) Use section type as option name and classifying filter as option value.
dhcp_option list of strings no (none) Additional options to be added for this network-id. :!: If you specify this, you also need to specify the network-id.
networkid string no (none) The network-id the classifier should apply to. Applies to all clients if left unspecified.

Section tag must be named, doesn't require option classifier and can include a list of dhcp_option and option force.

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docs/guide-user/base-system/dhcp.txt · Last modified: 2019/10/17 12:36 by vgaetera